1. Field of the Invention
This invention relates to a process for treating a waste liquor from hydrosulfite production process. More particularly, this invention relates to a process for treating the waste liquor comprising aerating the waste liquor to remove reducing inorganic sulfur compounds from the waste liquor and then applying an activated sludge process to the waste liquor. In this invention, by "hydrosulfite" is meant a alkali metal dithionite.
2. Description of the Prior Art
The waste liquor mentioned herein, is a waste liquor resulting from removal of a final product, anhydrous hydrosulfite, and a solvent from a reaction mixture in the process of producing anhydrous hydrosulfite by using sodium formate, alkali metal compounds, and sulfur dioxide such as the processes of U.S. Pat. Nos. 2,010,615 and 3,411,875 and British Patent No. 1,148,248. The waste liquor contains sodium thiosulfate, sodium sulfite, acid sodium sulfite, other unknown reducing inorganic sulfur compounds represented by the formula EQU Na.sub.x S.sub.y O.sub.z
where x, y and z are positive numbers, sodium formate and the like. Thus, if this waste liquor is directly disposed of, it becomes BOD and COD sources to cause water pollution. Therefore, some treatment should be applied before disposing of the waste liquor.
In general, the activated sludge process is known as a most effective method for treating waste liquor containing hydrophilic organic compounds and is used in various fields. For example, a waste liquor containing inorganic reducing substances such as sodium thiosulfate, sodium sulfite, and acid sodium sulfite and sodium formate can be effectively treated.
However, the present inventors have found that a conventional activated sludge process is not effective for treating hydrosulfite waste liquor. This seems to be due to unknown reducing substances present in the waste liquor from hydrosulfite process in addition to the above mentioned substances.
When a conventional activated sludge process is directly applied to the hydrosulfite waste liquor, BOD can not be sufficiently removed even if the waste liquor is diluted to a great extent, and further, the activated sludge itself becomes fine particles and is carried into the effluent without forming desirable floc, and therefore, a complete waste liquor treatment is not possible.